1. Field of the Invention
The present invention relates to rotary-vane pumps which may be utilized in vehicle power steering systems, and more particularly to a discharge rate control mechanism for such pumps for providing a desired discharge rate variation characteristic.
2. Description of the Prior Art
A pump of this kind is disclosed in U.S. Pat. No. 5,209,648. In this known pump, a flow control valve and a throttling insert are utilized for obtaining a delivery flow having a decreasing delivery tendency with rising rotational speed.
Another pump is known from Japanese Patent Provisional Publication No. 4-78076. This known pump has a pair of first and second discharge passages respectively provided with throttling means, a first flow control valve for controlling the pressure differential across each throttling means at a constant value, and a second flow control valve disposed in the second discharge passage for controlling the delivery flow therethrough in accordance with a back pressure which is acting on a vane and is variable with variation of rotational speed of the pump, i.e., the second flow control valve is movable in response to a back pressure acting on a vane for closing the discharge passage increasingly as the back pressure increases. By this, the rate of flow through the first discharge passage is regulated to a constant value irrespective of rotational speed of the pump, whereas the rate of flow through the second discharge passage decreases with increasing rotational speed of the pump. The second flow control valve however is not stable in operation since the fluid flow having passed the second flow control valve is partly delivered through a branch passage in which a spring for urging a valve spool in the direction to open the discharge passage is installed and further since the delivery flow through the branch passage is liable to be disturbed by the spring to apply a largely fluctuating fluid pressure on the spool.